don't know if that makes sense, but if [man://] is possible, [apropos://] is easy / note the
CGINVP:
apropos=1(→ http://www.freebsd.org/cgi/man.cgi?query=env&apropos=1)

At least the [man://] and [man1://] [man2://] ...
variants should be straightforward to implement.
The freebsd service seems to exist for several years, so there is a little bit of confidence that it will be available in the future.

I quite like the idea of the [man://...] and [man://program (1)] shortcuts.

I'm not sure about [man1://] - it's quite intuitive, but it goes against the current design of having one pseudo-protocol per link type. So I'd rather go with [man://program (1)] instead (but don't really mind either way).

As for the mskb links: I've never posted a link to one of them (but then again I don't use Windows either), and haven't seen particularly many here in posts or in the Chatterbox. But I don't think they would do any harm either, so I'm not against it either.

I really like this version best out of the ones so far proposed. I have a couple of suggestions that might improve it even more.

I really don't like anything made to look like a URL to have a double solidus (what W3 likes to use for "forward slash") in it. Yes, I know the format used for a shortcut wouldn't have to be directly related to the actual URL to which it refers. That would be very unlikely anyway unless someone at PM controlled the sites referenced. It's just a strong habit to eschew syntax in a URL-alike that wouldn't actually be valid for a URL. Perhaps one of these would be nicer for every section of a particular OS:

The question mark works for two reasons: it makes what comes after it just look like a query string, and it's a one character wildcard in many contexts (with most manual sections being represented by a single character). The asterisk is pretty plain: anything goes here. The "any" and "all" are pretty self-explanatory. The ASCII minus sign (doing double duty and standing in for dash here, as it often does) is just a common URL member character that happens to be a valid directory name (even if it's not straightforward to 'cd' to Bash) and valid URL segment.

Also, a simple one in your scheme since people have mentioned specific versions of the OS:

All of this leads to the inevitable maintenance questions, though. Who chooses which OSes are included? Who chooses which site to use for the man page links? What happens when that site goes down or changes its URL structure? Who's going to do the work on the PM end, and what Perl-related or PM-related resource would have had more attention if not for this? The MS Knowledge Base shortcut seems reasonable from the maintenance standpoint, as it's a single number reference to a single fairly stable site.

Better, but I think it would be more intuitive if the format for giving the section would be like [man://1 env] or [man://env(1)]: the first one is similar the the syntax of the command-line man utility which you invoke as man env or man 1 env, the second is like the header on the formatted manpages.

After fiddling around a bit with the FreeBSD man.cgi interface, I came up with a small heuristic (argl!) for specifying the OS and a Perl program that created the table below. It shall provide an impression of what the results might look like and summarise the feedback, I've got so far (thanks).

My assumptions:

the vast majority of users (of the minority that requires UNIX Man Pages anyhow) will need something like [man://program] only

optional (\D+) is a hint for an OS plus optional release (latter: ignored!); The FreeBSD site is quite good at guessing the current OS for a given manpath-attribute without a release number.

currently, the OS is simply uppercased (e.g. SUSE instead of SuSE)

[manX://abc] is offered as convenience shortcut for [man://abc (X)]

whitespace-tolerant, case-tolerant

class="man", class="mskb" for individual styling

using the format [man://prog (section) (os rel)] or [man://prog/section/os/rel] is mainly a matter of taste..., so I started with my favourite pattern

personally, I think the reference should contain no whitespaces

restriction of current implementation: no nested (...(..)..) or [..[..]..] allowed

handling of MSKB entries is trivial

....

Some testcases (including ill. formatted entries) below.
Some very simple heuristics are applied to find the manpath
attribute for a given Unix flavour keeping the maintenance overhead small at PerlMonks. If the heuristic fails, the FreeBSD site will fallback to the default OS (FreeBSD) - which is OK for me.
Although, abandoning the requirement to identify a certain OS would simplify the maintenance burden significantly.
The lookup-table is:

Update: Right, RHEL is not identical to RedHat Linux, but the intention was to come close to the family, which is rather RedHat than FreeBSD... However, it can be corrected by removing the rhel => 'redhat' entry from the hash above (see: mr_mischief's comment). Update2: If you like to experiment with the effect of the man and mskb styles, go to your Display Settings and add something to the
On-Site CSS Markup - section, e.g. (nobody says it has to look good!)